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What is the difference between symmetrical and unsymmetrical fault.

Subject: Power System Analysis

Topic: Unsymmetrical Fault Analysis:

Difficulty: Low

Symmetrical Fault :

• In symmetrical faults, all phases are shorted to each other or to earth (L-L-L)or(L-L-L-G).
• The nature of this type of fault is balanced.
• In this type of fault,fault currents in all phases are symmetrical i.e. their magnitudes are equal and they are equally displaced by angle 120o as shown in figure • It is more severe type of fault but it occurs rarely.
• For this reason balanced short circuit calculation is performed to determine these large currents.
• Because of balanced nature of fault, only one phase need to be considered in calculations since in the other two phases will also be similar.

Unsymmetrical fault:

• These faults involve only one or two phases.

• In this type of fault ,three phase lines become unbalanced.

• Those faults on the power system which gives rise to unsymmetrical fault currents(i.e, unequal line current with unequal phase displacement)are called Unsymmetrical fault.

• Such fault occur between line to ground and two lines.

• This type of fault mainly have two types
(a) shunt fault
(b) series fault.

• Shunt faults are Single Line to Ground fault (L-G) Fault, Line to Line fault (L-L Fault),Line to Line To Ground fault(L-L-G) fault as shown in figure below. • Example of Series fault is open conductor circuit.
• These faults mostly occurs on power system.
• The calculations of these currents are made by Symmetrical Component method.
• Unbalanced fault analysis is important for Relay setting, single phase switch and system stability studies.